It is known to those skilled in the art that there are heat developing photosensitive materials which enable images to be produced rapidly and readily by performing development in a dry process by heating. Such photosensitive materials and processes of producing images by the use thereof are disclosed, for example, in Japanese Patent Examined Publication Nos. 4921/1968 and 4924/1968, Fundamentals of Photographic Technology (Silver Salt Photography. Pages 553-555, Corona Publishing, 1979), and Research Disclosure No. 17029 (hereinafter abbreviated as RD. Pages 9-15, June 1978) and the like.
Generally, heat developing photosensitive materials can be divided into two groups, i.e., those which produce monochrome images and those which produce color images. Recently, studies have been made on the development of a heat developing color photosensitive material which produces color images by the use of dye-producing substances.
There have heretofore been proposed various processes of producing color images by using a heat developing color photosensitive material. Among them, there is a process in which color images are obtained by forming or releasing diffusible dyes by heat development, and the thus formed dyes are transferred onto an image-receiving element (this process will hereinafter be referred to as "transferring process"). This process, though it requires the use of an image-receiving element, is considered to be excellent in respect to the stability and vividness of obtained images, the simplicity of the developing procedure, as well as the accelerated development.
This kind of photosensitive material and the above-mentioned transferring process are disclosed, for example, in Japanese Patent Publication Open to Public Inspection (hereinafter referred to as Japanese Patent O.P.I. Publication) Nos. 12431/1975, 159159/1984, 181345/1984, 229556/1984, 2950/1985, 52643/1986, 61158/1986, 61157/1986, 180550/1985, 132952/1986, 139842/1986, U.S. Pat. Nos. 4,595,652, 4,590,154, and 4,584,367.
To promote heat development, heat solvents are often added to a photosensitive material. Especially in the case of the above-mentioned heat developing photosensitive material, various heat solvents or solvents are added to a photosensitive layer and/or non-photosensitive layer of the photosensitive material in order to attain a higher diffusible-dye forming efficiency or to improve dye-transferring efficiency.
The conventional heat solvents are roughly divided into two groups, i.e., solvents which are liquid at room temperature and solvents which are solid at room temperature but liquify when heat development is effected and exhibit various functions of a heat solvent. The representative examples of the former type of solvent include alcohols, polyols, phenols and ureas or amides of relative low molecular weight. However, this type of heat solvent is defective and, hence, is not suited to practical use: Specifically, many solvents of this kind are hygroscopic, and since they are present in liquid form in the photosensitive layer, they tend to make the photosensitive layer sticky. Further, the use of solvents of this kind inevitably causes photosensitive materials to adhere to each other or adhere to other materials.
On the other hand, in the case of the latter type of solvent, which is solid at room temperature, the above problems are eliminated to a considerable extent. As examples of this type of solvent, there can be mentioned compounds disclosed in Japanese Patent O.P.I. Publication Nos. 136645/1987, 232547/1985 and 53548/1988.
Meanwhile, the heat solvent which is solid at room temperature is required to be free not only from the above-mentioned problems of adhesion, but also from other problems concerning physical properties. For instance, heat solvents of this kind are required to exhibit the following advantageous properties:
(1) They do not volatilize nor gasify during the storage of a photosensitive material and during the process of heat development.
(2) They stably exist in the form of dispersed fine solid particles during the preparation of a photosensitive material.
(3) They retain their stable state of dispersed fine particles throughout the process of preparing a photosensitive material, and do not agglomerate.
(4) They do not exert any adverse effect on a hardened binder layer.
It is needless to say that, besides the above-mentioned properties, the heat solvent is required to play its essential role, i.e., to promote development as well as to increase the transferring efficiency of diffusible dyes.
However, all of the conventional heat solvents are unsatisfactory with respect to the above-mentioned properties. Under such circumstances, there is a strong demand in the art for a heat solvent which minimizes the above-mentioned problems.